Activity-dependent spinal cord plasticity affects motor function in health and disease. It contributes to skill acquisition, underlies the abnormal function associated with spinal cord injury, and should be a key factor in the design of effective new therapies. Yet this plasticity and the processes that create it are not understood. Progress requires a simple laboratory model in which it is possible to identify the sites and explore the mechanisms of activity-dependent spinal cord plasticity and to describe its translation into behavior. The spinal stretch reflex (SSR), or tendon jerk, which is mediated by a wholly spinal and largely monosynaptic pathway, is a model that satisfies these requirements. Because this spinal pathway is influenced by descending activity from the brain, monkeys, humans, and rats can gradually increase or decrease the SSR or its electrical analog, the H-reflex, in response to an operant conditioning paradigm. The learning changes the spinal cord, since evidence of it remains even after all descending activity is removed. This laboratory is defining the complex activity-dependent spinal cord plasticity underlying this simple change in motor function, the mechanisms that create the plasticity, and the manner in which it translates into behavior. The central hypotheses, supported by previous results and preliminary data, are: (1) that the complex spinal cord plasticity produced by H-reflex conditioning affects other spinal cord reflexes including reciprocal inhibition and presynaptic inhibition of agonist and antagonist muscles; (2) that the corticospinal tract, contralateral (and probably ipsilateral) sensorimotor cortex, and cerebellar-cortical connections are essential for acquisition and maintenance of this spinal cord plasticity; and (3) that this spinal cord plasticity affects spinal cord function during locomotion. These hypotheses will be tested by studying the effects of H-reflex conditioning on other spinal cord reflexes, by studying the effects of specific supraspinal lesions on H-reflex conditioning, and by studying the effects of H-reflex conditioning on responses to calibrated afferent inputs during locomotion. The results should lead to new understanding of the complex plasticity underlying the acquisition of motor skills and the changes in spinal cord function associated with trauma and disease, and should contribute to the development and assessment of new methods for improving function after spinal cord injury.